1. Field of the Invention
The present invention relates to an encoding control apparatus configured to encode a moving image.
2. Description of the Related Art
MPEG (Moving Picture Experts Group) standards are known as moving image compression encoding standards. An encoding apparatus for encoding a moving image to generate MPEG compatible data is adapted to determine a target amount of code for each picture on the basis of a target data rate. Based on the target amount of code for each picture, target amounts of code are averagely assigned to all macroblocks constituting each picture. Then, a quantization step by which each macroblock is encoded is determined so as to obtain the target amount of code, and the macroblocks are encoded.
One method to reduce the amount of code of a moving image is to skip some macroblocks as macroblocks that need not be encoded. For example, Japanese Patent Laid-Open No. 9-37268 discloses a method of determining a macroblock whose encoding is to be skipped on the basis of motion vector information, the maximum of absolute values of elements, and the sum of the absolute values.
However, this method has a problem. In a case where blocks surrounding a macroblock to be encoded are generally low-frequency blocks and the entire content of an image moves (due to panning, tilting, or the like), the macroblock to be encoded may be skipped if the difference between the macroblock to be encoded and a reference macroblock is small. Such skipping results from the presumption, made to minimize the amount of code, that the encoding target macroblock is equal to a macroblock located at a corresponding position in a reference frame. As a result, a macroblock remains unchanged between successive output frames, resulting in an unnatural looking image.